Being fit and well-built, performs extraordinarily lithe act (8)
I believe the answer is:
athletic
'being fit' is the definition.
(I've seen this before)
'extraordinarily lithe act' is the wordplay.
'extraordinarily' indicates an anagram (I've seen this in other clues).
'lithe'+'act'='litheact'
'litheact' with letters rearranged gives 'ATHLETIC'.
'and well-built performs' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for athletic that I've seen before include "Good at running and jumping" , "Physically fit, strong and active" , "Strong and active, good at sports" , "Strong and sporty" , "Given to partaking in sporting events" .)